When products of projections diverge

Abstract

Slow convergence of cyclic projections implies divergence of random projections and vice versa. Let $L_1,L_2,\dots ,L_K$ be a family of $K$ closed subspaces of a Hilbert space. It is well known that although the cyclic product of the orthogonal projections on these spaces always converges in norm, random products might diverge. Moreover, in the cyclic case there is a dichotomy: the convergence is fast if and only if $L_1^{\perp }+\cdots +L_K^{\perp }$ is closed; otherwise the convergence is arbitrarily slow. We prove a parallel to this result concerning random products: we characterize those families $L_1,\dots ,L_K$ for which all random products converge using their geometric and combinatorial structure.